Heat transfer method and means



Jan. 25, 1966 R. A. DAANE 3,230,636

HEAT TRANSFER METHOD AND MEANS Original Filed Aug. 28, 1959 2Sheets-Sheet 1 32 mvllmlmlm INVENTOR Raer'fd fizane ATTORNEYS Jan. 25,1966 R. A. DAANE 3,230,636

HEAT TRANSFER METHOD AND MEANS Original Filed Aug. 28, 1959 2Sheets-Sheet 2 BY f, I WATTORNEY;

vSXuS F29 kuuk 92x? R MV h k United States Patent 3,230,636 HEATTRANSFER METHQD AND MEANS Robert A. Daane, 1942 Greenview Drive, Eeloit,Wis. Continuation of application Ser. No. 836,683, Aug. 28, 1959. Thisapplication June 6, 1963, Ser. No. 285,911 5 Claims. (Cl. 34122) Thisapplication is a continuation of my application Serial No. 836,683,filed August 28, 1959, and entitled, Heat Transfer Method and Means, andnow abandoned.

The present invention relates broadly to impingement flow techniques,and is more particularly concerned with a novel method and apparatus fordirecting fluid flow normal to a heated or cooled surface in heattransfer relation therewith and having as an important illustrativeapplication the drying of newly coated paper sheets in a relativelyshort length run to prevent marring of the coated surface duringsubsequent passage over cylinder dryers or sheet roll-s.

It is an important aim of the present invention to provide a method ofimpinging air flow against the surface to heat or cool the same, andwhich is not restricted to a particular surface but has diverseapplications in the treatment of textile sheets, paper webs, plasticfilms, granular materials, fluid media and other surfaces.

Another object of this invention lies in the provision of a heattransfer method particularly effective in drying operations and whereina plurality of relatively small diameter air streams are impingedagainst the body to be dried from a direction normal or perpendicularthereto.

Another object of this invention is to provide drying apparatus for usein the papermaking art, and which comprises means defining a pluralityof circumferentially spaced plenum chambers relatively closely radiallyspaced from a paper web during wrapping travel about a cylindricaldryer, each plenum chamber having as one wall thereof a perforated platefacing the Wrapping web and providing a plurality of relatively smalldiameter impinging air .streams effective to reduce the web moisturecontent.

Still another object of the present invention lies in the provision ofnozzle means for a dryer head and taking the formof a plate memberhaving a plurality of rounded entrance holes spaced therethrough and ofa particular diameter and open area as compared with the plate totalarea to assure maximum performance efliciency in the drying operation.

A still further object of the instant invention is to provide a dryingmethod in which a plu-nality of spaced air streams of relatively smalldiameters are directed normal to a surface containing moisture to beremoved therefrom and in which the moisture bearing air streams aredirected in relatively smooth exit flow paths without substantialinterference with the impinging air streams.

Other objects and advantages of, the inventionwill become more apparentduring the course of the following description, particularly when takenin connection with the accompanying drawings. 7 p i In the drawings,wherein like numerals design-ate like parts throughout the same:

-FIGURE 1 is a somewhat diagrammatic view of a dryer section of a papermachine illustrative of a particular environment for the presentinvention;

3,23%,536 Patented Jan. 25, I956 FIGURE 2 is a vertical sectional viewof drying means constructed in accordance with the principles of thisinvention and positioned adjacent a conventional drying cylinder;

FIGURE 3 is a fragmentary plan view of nozzle means embodying theconcepts of this invention;

FIGURE 4 is a sectional view of the nozzle means of FIGURE 3, andshowing in detail the rounded entrance openings therethrough;

FIGURE 5 is a graph of surface temperature for free surface moisturedrying in which combined air and cylinder drying are employing as inFIGURE 2;

FIGURE 6 is a graph plotting index of performance against percent openarea, and showing that heat transfer performance is generally at amaximum when the perforations in the nozzle means of FIGURES 3 and 4 isabout 1.5 percent of the total area of the nozzle means;

FIGURE 7 is a graph plotting air mass flow rate, pressure drop andtheoretical pumping power against the heat transfer coefiicient, andillustrative of the performance of the drying means of this invention;and

FIGURE 8 is a graph plotting moisture content in a particular sampletested against drying time and showing the evaporation rates obtained bypractice of this invention.

It has long been generally recognized that impingement flow, or flowdirected normal to a heated or cooled surface is an eflicient means ofobtaining high rates of heat transfer between a fluid and a solid body.This is particularly true for heat transfer from or to a gas such asair. In

recent years, this means of heat transfer has begun to be used fordrying of paper, particularly in the drying of coated paper where theproblem arises of having to dry the newly coated sheet in a relativelyshort length of sheet run, so that it. can be passed over subsequentcylinder driers or sheet rolls without marring the coated surface.

Various drying devices have been proposed for use with paper makingmachines, and one known form comprises generally clamshell-shapedheaders supporting slotted nozzle means spaced from the paper sheet asit is wrapped about suitable rollers. Another form of drying devicesuggested comprises a plurality of generally vertically aligned andhorizontally spaced slender perforated tubes arranged to sweepconditioned air against the sheet during travel through a so-calledconditioning chamber housing drying cylinders. v

The described slot arrangement and means of the same general characterhas not, however, been characterized by eflicient performance asmeasured by the heat transfer coeificient obtainable for a givenexpenditure of air blower horsepower. Additionally, known nozzle slotarrangements have the disadvantages of interfering with the impingementflow and generally require a relatively complex system of air removalducts between the slots. Further, in any arrangement in whichslot-shaped nozzles formed by sheet metal vanes projecting into thespace between the drier and the impingement air supply plenum to permitspent air removal with a flow between and parallel to-the slot,introduces dimculties in the practical operation of a paper drier sincethe sheet metal nozzles are not generally dimensionally stable andreadily trap sheet fragments should a paper break at the drier occur.Projection of the nozzles into the impingement air space is of coursenecessary in order to make the lateral passages for exhaustion flowsufliciently large so that excessive velocity does not result when thedrier is of the width required on modern paper machines, while at thesame time keeping the desired ratios between nozzle throat width, nozzletip distance from the sheet, and number of nozzles per inch.

Referring now to the drawings, there is shown somewhat diagrammaticullyin FIGURE 1 a dry end section -of drying apparatus of a paper machinewith which the instant invention is of important application. However,it will be readily apparent during the course of the description now tofollow that the drying means and method herein disclosed has numerousapplications other than for paper machine driers, and is productive ofadvantageous results in the heating or cooling of any surface by meansof air. Exemplary of such additional applications are textile drying,drying of plastic films, use of the apparatus and method with slasherdriers, potato and cereal drying, milk drying and related processeswhich can clearly benefit by the inventive concepts herein disclosed,since the heat transfer behavior in the named applications isessentially the same as that existing in the drying of paper sheets orwebs.

The dry end section 10 shown in FIGURE 1 comprises a first row ofhorizontally aligned drying cylinders or drums Illa-d of which four areshown in the exemplary embodiment illustrated, and a second row ofhorizontally aligned drying cylinders 12a-d staggered with respect. tothe drying drums 11 of the first row. A web of paper W passesalternately about the drying cylinders 11 and 12, and is maintained incontact with the surfaces of the drying cylinders 11a and 11b by a felt12, and against the surfaces of the drying drums 12a-d by a felt 14. Asuitable number of rollers 15 are arranged to guide the felt 13, androllers 16 guide the felt 14. The drying arrangement described will berecognized as of essenti'ally conventional construction, and inperformance of the drying cycle, the cylinder or drum 11d may be a sweatdrier supplied with relatively cold water to cause condensation on thesurfaces thereof from surrounding moist atmosphere, the condensate beingpicked up by the web W traveling over this drier to restore a portion ofthe moisture removed by any over-drying. However, it is of courseunderstood that all applications do not require use of the sweat drier11d.

Impingement air drying means provided by this invention, and having theadvantages earlier noted, is designated generally by the numeral 17,.and it may be seen from FIGURES 1 and 2 that such. means are located inrelatively close wrapping relation to the drying cylinder 11c. Identicalmeans could of course additionally be employed in association with thedrier drum or cylinder 12d, and in such an application the felt 14 wouldbe removed from the latter drum.

The impingement air drying means 17 comprises a housing or insulatedhood cover 18 having a length preferably coextensive with the width ofthe web W and provided with a bottom opening 19 therein, so that whenthe housing 18 is suitably supported it overhangs the cylinder 11capproximately to the distance shown. Stated otherwise, the housing 18extends a sufh-cient distance aboutv the cylinder or drum 116 to becoextensive with the wrap of the web W during travel along thecircumferential portion of the rotatable drum 11c. The housing 18 may beshaped to provide a pair of generally upright opposed side Walls 20 and21, a pair of opposed end walls 22 (one of which is shown), a generallyflat top Wall 23, and a downwardly inclined bottom wall 24 which may behanged at 25 to provide a confined path of web travel during on-runningand off-running wrapping travel along the drum 11s.

The housing 18 supports therewithin in any suitable manner a pluralityof circumfcrentially spaced supply plenums 26ad, four of which are shownin the exemplary form illustrated. Each supply plenum 26a-d' may be ofgenerally semicylindri-cal shape, and the plenum means 26a-dcommunicate, respectively, with conduit means 27ad leading to a supplyduct 28, which in turn connects with a suitable source of heated orcooled air. To exhaust moisture bearing air from the housing 18, aplurality of exhaust openings 29 are provided, these openings beingsomewhat diagrammatically shown and communicating with an exhaust fan orthe like (not shown).

Each supply plenum 26 supports nozzle means Ella-d, which, in accordancewith the. principles of this invention, takes the form of a plate havinga plurality or array of spaced holes or perforations 31 therein of aparticular configuration, diameter and number in. order to obtainmaximum drying efliciency from the impingement air drying means 17. Theperforated plates 30 close the mouth portion of each supply plenum 26,and assure that the air streams issuing therefrom impinge against thepaper web W normal thereto. Each plate. 30 is of generally arcuate shapewhen viewed in end, or stated otherwise, each plate or nozzle member 30has a curvature corresponding to that of the cylinder or drum 11c.

Referring now also to FIGURES 3 and 4,. it will be observed that eachopening 31 in the perforated plate 30 has a rounded entrance portion 32which connects with a generally straight walled discharge. portion 33.In tests performed to date, which will be described hereinafter, it hasbeen established that a desirable range of diameters for the openings 31is between A and inch measured at the discharge portion 33, and anoptimum diameter at this location is /8 inch. A desirable radius on therounded entrance portion 32 has been found to be inch. Primarily forstructural reasons the perforated plates 30 each have a thickness ofapproximately /4- inch.

While the radius about which the rounded or flared entrance portion 32of the nozzle is. generated may be varied, it is of advantage tomaintain a distinct relationship between the ratio of the diameter. ofthe nozzle opening or orifice to the radius about which the roundedentrance portion 32 is struck. With a /s-inch diameter nozzle openingand a 7 -inch radius of the rounded entrance portion 32, the ratiov ofthe nozzle opening or orifice diameter to the radius of the roundedentrance. portion or flare at the inlet of the orifice equals one-half,and it has been found that it is advantageous to maintain this ratioregardless of changes indiameter of the nozzle opening as willhereinafter more clearly appear as this specification proceeds.

Moreover, there is distinct advantage in maintaining a fairly definiteratio between the radius of the rounded entrance opening or flare of thenozzle opening and the thickness of the plate 30. As previously stated athickness of approximately A of an inch is a practical thickness for theplate and the ratio of the radius of the rounded entrance portion orflare 32 to the thickness of the plate as herein described, is 1 /3 andit has been found advantageous to maintain this ratiobetween 1 and 1 /2.

In tests which have been conducted it has been demonstrated that anozzle plate employing %-inch diameter holes with a ;-inch radiusrounded entrance. is productive of a coefiicient of discharge for airflow through the. holes which is markedly higher than the coefficient ofdischarge obtained when square edged holes are pro-- vided.Specifically, rounded entrance holes as herein disclosed produce acoefficient of discharge, at jet velocities.

Where G=Mass flow rate of air, lb./sec. ft. based on hole area.

g=32.2 ft./sec.

=Density at exit pressure, lb./ft.

AP=Pressure drop, lb./ft.

The relatively low value of C for the square edged holes means an actualloss in performance when this type hole is used. While the low value ofC might be thought to be merely an indication of more contraction of theflow stream emerging from the square edged hole, if this were the casethe same performance as with rounded entrance holes could be achieved bymerely increasing the hole size of the square edged holes. However, thiswas actually tried with plates provided with square edged holes havingthe same total air flow versus pressure drop curves as correspondingrounded entrance hole plates. The square edged hole plates were testedfor heat transfer per-formance, and except in those cases where verylarge percentages of open area were provided, the index of performanceWas smaller for the square edged holes by at least percent.

Work performed to date has also demonstrated that a desirable spacing ofeach perforated plate 30 from the web W during its Wrap about the drum110 is one inch. This provides sufficient clearance to allow spent airremoval without excessive lateral velocity or pressure drop, even whenthe span of the perforated plate sections 30 or the distance between theexhaust ports 29 is as large as 24 inches. In addition, the one inchspacing leads to an optimum hole size and hole spacing which are smallenough so that the possibility of spotty drying is not present. Further,the spacing of one inch also appears to be desirable from the standpointof possible sheet wrap, in the event of a paper break, on the cylinderdrier 110 over which the air drier 17 is placed. While the one inchspacing is desirable for the reasons stated, the spacing can of coursebe altered, however, it has been found when the one inch clearance isincreased to two inches, a loss of between 4 and 6 percent in heattransfer coefiicient is produced, using the same air velocity, the sameratio of clearance to hole size, and the same percentage' of open area.

Hole size or diameter as measured across the straight through portion 33also has an efiect upon index of performance, the manner of obtainingthis index being referred to in detail hereinafter. With respect to holesize, it was found that the index of performance reaches a maximum valuewhen the hole size is between inch and inch, and in order to permit someincrease in the one inch spacing of the plate from the web without lossin performance, the larger hole size of /8 inch is now preferred.

The effect of open area is, however, a much more influential parameterupon the index of performance. Percentage of open area refers to a ratioof hole or open area to total area of the perforated plate 30, and theeffect of variations in the percentage open area is clearly demonstratedin FIGURE 6. As shown therein, a rather sharp increase in index ofperformance occurs at the point at which the open area is approximately1.5 percent although this curve shows that satisfactory performance isobtained in the range of 1.4% to 1.6%. The values plotted in the graphof FIGURE 5, were obtained utilizing,0.375 inch diameter roundedentrance holes in a 0.25- inch plate spaced one inch from the surface.

The index of performance of the impingement air system was obtained inthe following manner. In general, in forced convection, heat transfer isrelated to flow by:

where.

h =Heat transfer coefficient from air to sheet surface, B.t.u./hr./ft.F. (per unit area of impingement hood coverage).

C =A eoeflicient which depends on the shape and size of the impingementsystem.

u=Fluid viscosity, lb./ hr. ft.

k=Conductivity of the fluid, B.t.u./hr. ft. F..

G=Mass flow rate, lb./hr. ft. (based on total surface area covered bythe impingement system).

m, n=Constants.

An impingement air system involves flowing air at high velocity throughnozzles or orifices with an eventual zero pressure recovery of thekinetic energy of the jet, so that the pressure drop (or impingementsupply air plenum pressure) is given by:

AP=C G uPSI Where v=specific volume of the air, ft. /lb.

and C is a dimensional constant which depends on the nozzle design andon the ratio of nozzle area to total impingement system coverage area.

For the relatively small changes in absolute pressure of the air inthese systems, the pumping power (not corrected for duct losses and pumpefficiency) will be approximately:

hp.=unit power=7.27 x10" GvAP hp./ft. Where the 7.27 10 is a unitsconversion factor.

This can be combined with the two equations above to give:

The factor, C is a combination of constants C C2, and the fluid propertyterms. For a given fluid, in the present case air at a fixedtemperature, C becomes the index of performance of an impingement airsystem.

Referring now again to FIGURE 3, it may be seen that the holes oropenings 31 in each plate 30 are preferably arranged in groups of threewhen the plate is viewed in plan, and the groups of openings desirablyform an equilateral triangle which is slightly askew from the sheettravel direction indicated by the arrow applied to FIG- URE 3. Asearlier noted, the preferred spacing of the perforated plates 30a-d fromthe circumference of the drum 11c is approximately one inch, andinvestigations have shown that there is no particular advantage in arelatively closer spacing between the nozzle tips and the cooled orheated surface. Specifically, the average heat transfer coeflicient fromthe air to sheet surface varied only within the narrow range of 50 to 53B.t.u./hr. ft. F. when the nozzle throat diameter was varied from 0.31to 0.125 inch and the distance of the plate from the surface varied from0.375 inch to 1.50 inches. More specifically, the value of the heattransfer coefficient was 50 with a nozzle throat of 0.31 and a platedistance of 0.375, a. value of 51 obtained when the nozzle throat was0.62 inch and the plate distance 0.75, a heat transfer 00- eflicient of53 was found when the nozzle throat was 0.94 inch and the plate distance1.125 inches, and the coefli-cient was 50 when the nozzle throat was0.125 inch and the plate distance 1.50 inches. As indicated, the heattransfer coefficient is expressed in B.t.u./hr. ft? F..

It is to be seen from the foregoing data that close clearance betweenthe impingement nozzles and the heated or cooled surface is not animportant requirement for obtaining an optimally designed impingementair heat transfer system. However, the importance of the preferred oneinch spacing is that the spent air can be removed from the space betweenthe perforated plate and the heated or cooled surface with a flow of thespent air in any direction parallel to the perforated plate, withoutinterfering appreciably with the impingement flow. The path of exhaustair flow is shown in FIGURE 2 by arrows, and it is to be noted that saidflow is essentially parallel as described. Accordingly, the smoothperforated plate 30 of this invention provides no interference withimpingement flow, which is the case when slots are employed since theynecessarily must be placed at 90 to the direction of sheet travel, forreasons of drying uniformity across the width of the sheet. Removal ofthe spent air when a slotted arrangement is used, accordingly requires acomplex system of air removal ducts between the slots, or aconfiguration of sheet metal nozzles projecting into the space betweenthe drier and the impingement air supply plenum, in order that spent aircan be removed with a flow in between and parallel to the slots. Asearlier noted, this introduces many practical diflicul ties and createsconditions interfering with eflicient paper machine operation.

The spacing of the nozzle openings 31 along the plate 30 in a triangulararray provides a nozzle pattern in which lines drawn along the bottom ofthe plate 30 transversely of the direction of web travel along the drierdrum 110 to intersect one nozzle opening 31 will at all times intersectmore than one nozzle opening, providing a uniform distribution of airthrough the plate 30 and thereby resulting in a uniform drying rate ofthe web across its entire Width. This is particularly important whenfreshly coated webs are being dried and the coating material is stillmore or less fluid, making uniform drying treatment essential.

In FIGURE 7 of the application drawings there are shown curves whichclearly illustrate the improved performance of the impingement airdrying means of this invention. The curves presented therein plot heattransfer coeflicient against air mass flow rate per square foot of driersurface covered by the impingement plates (in units of c.f.m., of 70 F.,atmospheric air), pressure drop in p.s.i. (for flow through theperforated plate and to the hood exhaust ports 29) and theoreticalpumping power in hp./ft. as given hereinabove. The curves of FIGURE 7are readily employed in connection with the graph of FIGURE 5, showingsurface temperature for free surface moisture drying using a combinedair and cylinder drying, and utilization of the graphs of FIGURE 5 andFIGURE 7 permits prediction of drying rates and supplies informationallowing a particular air system to be sized for specified requirements.

Drying tests have been conducted to demonstrate the eflicientevaporation rates obtained by the apparatus of this invention. For thispurpose, 12-inch by 4-inch newsprint samples were dried by means of hotair impinged thereon through perforated plates 30 essentially 'as shownin the instant drawings. The samples had not been dried and rewetted,but were in the form as received from customary press sections. Dry airat measured temperature and velocity was impinged on the sheet samplesfor short periods of time, which was controlled by an electrically timedsolenoid valve. The results of one test are shown in FIGURE 8, whichplots drying time in seconds against the weight of moisture per pound ofdry paper. The weight of news-print was 0.0107 pound per square foot,the air flow was 112 c.f.m. standard condition air per square foot, andthe air temperature was 300 F. The results obtained were at least equalto those obtained by the other drying means. However, it may be notedfrom the graph of FIGURE 8 that the fall-off in drying rate is less withimpingement air drying as herein disclosed, when compared with hotsurface drying.

A preferred impingement plate configuration has been illustrated anddescribed, and as well as the hole configuration, percent open area,hole diameter, and spacing of the plate from the heated or cooledsurface However, while in work performed to date the describedreiationships have proven to be productive of substantially improvedresults, it is of course apparent that various changes and modificationsmay be effected without departing from the novel concept of the presentinvention.

In the claims:

1. An apparatus for drying a fibrous web traveling about a rotatabledrying cylinder, comprising a hood supported adjacent the dryingcylinder and extending about a portion of the circumference thereof, atleast one plenum chamber contained within said hood, an air duct leadinginto said hood and plenum chamber, an air outlet leading from said hood,for withdrawing spent moisture bearing air from said hood and from thefibrous Web passing about the drying cylinder, a plate conforming to theperiphery of said drying cylinder and spaced from said drying cylinder adistance in the range of between one and two inches and forming a bottomwall of said plenum chamber and having a plurality of nozzle openingsleading therethrough, each of said nozzle openings consisting in aperforation through said plate and having an inwardly flared entrancethroat in communication with said plenum chamber and a generallycylindrical wall leading therefrom for impinging air onto the webpassing about said drying cylinder, said nozzle openings providingapproximately a 1.5% open area in said plate, the diameters of saidcylindrical wall portions of said nozzle openings being substantiallyinch and the radii about which the flares of said flared throats aregenerated being inch.

2. An apparatus for drying a fibrous web traveling about a rotatabledrying cylinder, comprising a hood supported adjacent the dryingcylinder and extending about a portion of the circumference thereof, atleast one plenum chamber contained within said hood, an air duct lead nginto said hood and plenum chamber, an air outlet lead ng from said hood,for withdrawing spent moisture bear ng air from said hood and from thefibrous web passingabout the drying cylinder, a plate conforming to theperiphery of said drying cylinder and spaced from said cylinder adistance of at least one inch and not over two inches and forming abottom wall of said plenum chamber and having a plurality of nozzleopenings leading therethrough, each of said nozzle openings being nolonger than the thickness of the plate and consisting in an unimpededperforation through said plate and having a diameter in the rangebetween /1 inch and of an inch and having an inwardly flared entrancethroat in communication with said plenum chamber, the radius about whichthe flare of said flared entrance throat is generated "beingsubstantially 7 of an inch, said nozzle openings having a generallycylindrical Wall leading from said flared entrance throat for impingingair onto the web passing about said drying cylinder, and said nozzleopenings provrding substantially a 1.5 open area in said plate.

3. An apparatus for drying a fibrous web traveling about a rotatabledrying cylinder, comprising a hood supported adjacent the dryingcylinder and extending about a portion of the circumference thereof, atleast one plenunr chamber contained within said hood, an air ductleading into said hood and plenum chamber, an air outlet leading fromsaid hood, for withdrawing spent moisture bearing air from said hood andfrom the fibrous web passing-about the drying cylinder, a plateconforming to'the periphery of said drying cylinder and spaced from saidcylinder a distance of at least one inch and not over two inches andforming a bottom wall of said plenum chambet and having a plurality ofnozzle openings leading therethrough, each of said nozzle openings beingno longer than the thickness of the plate and consisting in an unimpededperforation through said plate and having a diameter in the rangebetween 4 inch and /8 of an inch and having an inwardly flared entrancethroat in communication with said plenum chamber, the radius about whichthe flare of said flared entrance throat is generated beingsubstantially of an inch, said nozzle openings having a generallycylindrical wall leading from said flared entrance throat for impingingair onto the web passing about said drying cylinder, said nozzleopenings being arranged in a generally equilateral triangular array, allof the sides of the triangles of which triangular array are angular bothwith respect to the direction of travel of the web and with respect to aline extending perpendicular .thereto, and providing a nozzle patternuniformly distributed about said plate and having an open area ofsubstantially 1.5% of the total area of said plate.

4. A drying apparatus in accordance with claim 2 wherein the ratio ofthe diameter of each discharge orifice to the radius of the roundedentrance throat of each orifice is at least /2.

5. A drying apparatus in accordance with claim 2 wherein the ratio ofthe radius of the rounded entrance throat of each nozzle opening, to thethickness of said plate is one or greater and is not more than 1 /2.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTSFrance.

WILLIAM F. ODEA, Acting Primary Examiner.

15 NORMAN YUDKOFF, Examiner.

2. AN APPARATUS FOR DRYING A FIBROUS WEB TRAVELING ABOUT A ROTATABLEDRYING CYLINDER, COMPRISING A HOOD SUPPORTED ADJACENT THE DRYINGCYLINDER AND EXTENDING ABOUT A PORTION OF THE CIRCUMFERENCE THEREOF, ATLEAST ONE PLENUM CHAMBER CONTAINER WITHIN SAID HOOD, AN AIR DUCT LEADINGINTO SAID HOOD AND PLENUM CHAMBER, AN AIR OUTLET LEADING FROM SAID HOOD,FOR WITHDRAWING SPENT MOISTURE BEARING AIR FROM SAID HOOD AND FROM THEFIBROUS WEB PASSING ABOUT THE DRYING CYLINDER AND SPACED FROMING TO THEPERIPHERY OF SAID DRYING CYLINDER AND SPACED FROM SAID CYLINDER ADISTANCE OF AT LEAST ONE INCH AND NOT OVER TWO INCHES AND FORMING ABOTTOM WALL OF SAID PLENUM CHAMBER AND HAVING A PLURALITY OF NOZZLEOPENINGS LEADING THERETHROUGH, EACH OF SAID NOZZLE OPENINGS BEING NOLONGER THAN THE THICKNESS OF THE PLATE AND CONSISTING IN AN UNIMPEDEDPERFORATION THROUGH SAID PLATE AND HAVING A DI-